Various kinds of offset printing plate precursors for directly producing printing plates have hitherto been proposed, and some of which have already been put into practical use. A widely employed precursor is a light-sensitive material having a photoconductive layer comprising photoconductive particles such as zinc oxide particles and a binder resin provided on a conductive support. A highly lipophilic toner image is subsequently formed on the photoconductive layer surface by an ordinary electrophotographic process. The surface of the photoconductive layer having the toner image is then treated with an oil-desensitizing solution, called an etching solution, to selectively render the non-image areas hydrophilic thereby producing an offset printing plate.
In order to obtain satisfactory prints, an offset printing plate precursor or light-sensitive material must faithfully reproduce an original on the surface thereof; the surface of the light-sensitive material should have a high affinity for an oil-desensitizing solution so as to render non-image areas sufficiently hydrophilic and, at the same time, should be water resistant. When used as printing plate, the photoconductive layer having a toner image formed thereon should not come off during printing, and should be well receptive to dampening water so that the non-image areas can remain sufficiently hydrophilic to be free from stains, even after a large number of prints have been reproduced from the plate. These properties are greatly affected by a binder resin used in the photoconductive layer as already known. With respect to offset masters, various binder resins for zinc oxide have been investigated particularly for the purpose of improving the oil-desensitivity. Specifically, copolymers containing at least methacrylate (or acrylate) components, for example, those described in JP-B-50-31011 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-53-54027 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-57--202544 and JP-A-58-68046 are known.
On the other hand, resins of the type which contain functional groups capable of producing hydrophilic groups through decomposition have been investigated on an aptitude for the resin binder. For example, the resins containing functional groups capable of producing carboxy groups through decomposition as described in U.S. Pat. Nos. 4,792,511 and 4,910,112 and JP-A-62-286064, and the resins containing functional groups capable of producing carboxy groups through decomposition and having a crosslinking structure therebetween which restrains the solubility thereof in water and impart water swellability thereto, whereby the prevention of background stains and the printing durability are furthermore improved as described in U.S. Pat. Nos. 4,960,661 and 5,017,448 are known.
However, when these resins are practically employed as binder resins for lithographic printing plate precursors, they are still insufficient with respect to the background stains and printing durability.
Moreover, addition of resin grains containing functional groups capable of producing carboxy groups through decomposition to the photoconductive layer for the purpose of improving the water retentivity is described in U.S. Pat. No. 4,971,870.